1. Field of the Invention
The present invention relates to a technique for relative positioning between a mask pattern and a photosensitive substrate, which technique is applied to an exposure apparatus used in a photolithography process for exposing the mask pattern onto the photosensitive substrate in manufacturing, e.g., semiconductor elements and, more particularly, to a technique for detecting the mark pattern on the photosensitive substrate.
2. Related Background Art
In the photolithography process for manufacturing, e.g., semiconductor elements, liquid crystal display elements, or thin-film magnetic heads, an exposure apparatus is used to transfer an image of a photomask hereinafter) having a transfer pattern onto a photoresist-coated wafer (or a photosensitive substrate such as a glass plate) in accordance with a projection exposure method through a projection optical system or proximity exposure method.
In this exposure apparatus, positioning (alignment) between a reticle and a wafer must be performed with high precision prior to exposure. A position detection mark (alignment mark) obtained by exposure, transfer, and etching in the previous process is formed on the wafer. The accurate position of the wafer (i.e., a circuit pattern on the wafer) can be detected by detecting the position of this alignment mark.
In recent years, there has been proposed a method of forming a one-dimensional or two-dimensional grating mark on a wafer (or a reticle), projecting two coherent beams symmetrically inclined in the pitch direction on the grating mark, and causing two diffracted light components generated from the grating mark in the same direction to interfere with each other to detect the position and offset of the grating mark in the pitch direction, as disclosed in Japanese Patent Application Laid-open Nos. 61-208220 (corresponding U.S. Pat. No. 4,828,392; to be referred to as reference (A) hereinafter), 61-215905 (corresponding U.S. Pat. No. 4,710,026; to be referred to as reference (B) hereinafter), and the like. Reference (A) discloses a homodyne scheme in which two symmetrical coherent beams have the same frequency, while reference (B) discloses a heterodyne scheme in which a predetermined frequency difference is present between two symmetrical coherent beams.
Heterodyne schemes in which position detection devices of these schemes are applied to a TTR (Through-The-Reticle) alignment system and a TTL (Through-The-Lens) alignment scheme are proposed in Japanese Patent Application Laid-open Nos. 2-227602 (corresponding U.S. Ser. No. 483,820 filed on Feb. 23, 1990, now U.S. Pat. No. 5,489,968 issued Feb. 6, 1996; to be referred to as reference (C) hereinafter), 3-2504 (corresponding U.S. Pat. No. 5,118,953; to be referred to as reference (D) hereinafter), and the like. In the heterodyne schemes disclosed in these references (C) and (D), an Hexe2x80x94Ne laser beam is simultaneously incident on two acousto-optic modulators (AOMs), and the AOMs are driven by high-frequency drive signals (one drive signal: 80 MHz; the other drive signal: 79.975 MHz) having a frequency difference of, e.g., about 25 kHz, thereby imparting the frequency difference of 25 kHz to diffracted beams emerging from these AOMs. These two diffracted beams serve as a pair of incident beams for irradiating a grating mark on a wafer or reticle at a predetermined crossing angle.
In the heterodyne scheme, the frequency difference (25 kHz) between the two incident beams is given as a reference AC signal, a phase difference between the reference AC signal and a signal obtained by photoelectrically detecting an interference light beam (beat light beam) of two diffracted light components generated from the grating mark is measured, and this phase difference is detected as a position offset (positional deviation) amount from the reference point in the pitch direction of the grating mark.
According to the heterodyne scheme described above, when the two incident beams for illuminating the grating mark have better monochromaticity, the detection precision in position offset, i.e., a resolution can increase. Position detection and positioning on the nanometer order can be performed. Excellent monochromaticity in the two incident beams indicates that the phase on the wavelength order between various diffracted light components generated by the grating mark tends to be sensitively changed by asymmetry of the grating marks, a resist layer, and the like.
An influence of the resist layer cannot be inevitably avoided in wafer alignment in an exposure apparatus. Unless a special technique for locally removing a resist from a mark portion is used, or unless an optical mark detection technique is given up, this problem is left unsolved.
A heterodyne scheme capable of reducing the influence of the resist layer and an influence of asymmetry in the sectional shape of the marks and allowing more accurate position detection is proposed in Japanese Patent Application Laid-open No. 6-82215 (corresponding U.S. Ser. No. 091,501 filed on Jul. 14, 1993; to be referred to as reference (E) hereafter). According to a technique disclosed in this reference (E), a plurality of beams having different wavelengths, or a white beam is used, and two diffracted beams obtained upon irradiation of the plurality of beams or the white beam on a stationary diffraction grating are incident on the first AOM. 0th-, +1st- and xe2x88x921st-order beams diffracted by the first AOM are relayed to cross each other in the second AOM, thereby obtaining a pair of incident beams having the first wavelength and a pair of incident beams having the second wavelength. These two pairs of incident beams are simultaneously projected onto the grating mark on the wafer.
In this case, an interference beat light beam generated by the grating mark and photoelectrically converted include the first wavelength component and the second wavelength component. These components are photoelectrically detected in the form of a sum as a light amount on the light-receiving surface of a photoelectric element. For this reason, the mutual phase differences of the interference beat light beams of the respective wavelength components caused by the influence of the thin-film interference of the resist layers and the influence of asymmetry in the sectional shape of the marks can be averaged in terms of light intensity. Therefore, more accurate position detection can be performed.
Regardless of the homodyne and heterodyne schemes, a light source suitable for obtaining a multi-wavelength illumination light beam is generally selected from light sources having a high coherency and a sufficiently large light intensity, such as a gas laser light source or a semiconductor laser light source. For this reason, wavelengths selected for a multi-wavelength light beam in the conventional arrangement are determined such that the oscillation center frequency is shifted by an appropriate amount, e.g., 20 nm to 40 nm from practical light sources (e.g., laser light sources having excellent records of performance).
The surfaces of the grating marks on the wafer are coated with a resist layer having an almost predetermined thickness (e.g., about 0.5 xcexcm to 1.5 xcexcm). This thickness varies on a given central value. In addition, the thickness of the resist layer varies depending on the positions on the wafer. The sectional shape (small step differences of the grating grooves) of the grating marks slightly changes depending on the positions on the wafer in accordance with a wafer process. The total amplitude reflectance of the portion including the grating marks and the resist layer on their surfaces often greatly varies with respect to an illumination light beam which is converted as a multi-wavelength illumination light beam with a specific wavelength component.
Wavelength selection for forming a multi-wavelength illumination light beam cannot always be optimized so as to achieve high-precision position detection for a grating mark whose amplitude reflectance can greatly vary.
Assume that an illumination light beam used for position detection is formed into a beam having a plurality of wavelengths or a beam having a predetermined wavelength bandwidth, and that interference light beams including a plurality wavelength components generated by the grating mark are simultaneously received by a single photoelectric element. In this case, if the illumination light beam includes a high-intensity wavelength component, the interference light beam from the grating mark is increased at this wavelength component, thus often posing a problem to obtain an averaging effect. In addition, even if the intensities of the wavelength components in the illumination light beam are identical to each other, large differences may occur for the wavelength components of the interference light beam from the grating mark depending on the surface state (e.g., the irregularity in thickness of the resist and the degree of asymmetry of the grating marks) on the photosensitive substrate such as a wafer.
Even if an interference light beam having a plurality of wavelength components generated by the grating mark is received by a single photoelectric element, high precision of position detection cannot always be obtained depending on the surface state of the substrate.
A heterodyne system in which the influence of a resist layer and the influence by the deformation of the shape (such as the asymmetry of the cross-sectional shape) of a grating mark are reduced to thereby make more accurate position detection possible is also proposed by U.S. Pat. No. 5,160,849 (H).
In this publication (H), there is disclosed a technique of finding a first position offset amount of the grating mark measured on the basis of the photoelectric signal of the interference beat light of xc2x11st-order diffracted lights created perpendicularly from a grating beam and a second position offset amount of the grating mark measured on the basis of 0-order diffracted light and xc2x12nd-order diffracted lights created from the grating beam, and selecting one of the two positional deviation amounts.
Also in a position detecting system of the homodyne type, as disclosed in Japanese Patent Application Laid-open No. 61-208220 (I), there is proposed an attempt to know the asymmetry of a diffraction grating by comparing the magnitudes of the light intensities of a plurality of high-order diffracted lights created from a diffraction grating on a substrate.
However, in the system of the publication (I) wherein as in the prior art, a variation in the distribution by the each of the intensities of high-order diffracted lights created from a grating mark by the application of an illuminating beam of a single wavelength is presumed, a number of data bases for making various conditions such as the optical non-uniformity of a resist layer covering the grating mark and the asymmetry of the cross-sectional structure of the grating mark and the variation in the distribution of the intensities of the high-order diffracted lights correspond to each other become necessary.
Also, in the system as disclosed in the publication (H) wherein one of the first position offset amount measured on the basis of the coherent light of 1st-order diffracted components from the grating mark and the position offset amount measured on the basis of the coherent lights of 0-order and 2nd-order diffracted components, there has been the problem that the deterioration of the position offset detection accuracy by the optical non-uniformity of the resist layer and the influence of the asymmetry of the cross-sectional structure of the grating mark cannot be synthetically suppressed (the improvement in the accuracy becomes limited).
Also, in connection with the technique of the publication (H), it would occur to mind to average the measured first position offset amount and second position offset amount with weight given thereto in conformity with the intensities (the degrees of modulation) of the respective interference lights, but it may sometimes be impossible to attain an improvement in accuracy simply by the averaging having added thereto the simple weight conforming to the degrees of modulation of the interference lights from the grating mark on a wafer. This is considered to be caused by the fact that the amplitude reflectance of the surface portion of the wafer including the grating mark for the illuminating beam changes variously for each wafer and at each location (shot area) on the wafer.
It is, therefore, an object of the present invention to provide a position detection method and apparatus almost free from the influence of a surface state of a periodic grating pattern (grating marks) formed on a substrate such as a wafer.
It is another object of the present invention to provide a position detection method and apparatus capable of adaptively changing the time coherence of a multi-wavelength illumination light beam with changes in sectional shape (e.g., a groove depth) of grating marks and thickness of a resist when an illumination light beam for detecting the grating mark on a substrate is formed as a multi-wavelength light beam using coherent beams having relatively narrow spectral widths radiated from a plurality of light sources.
It is still another object of the present invention to provide a projection exposure apparatus capable of generating an incoherent multi-wavelength illumination light beam using a plurality of coherent light sources and using this illumination light beam for alignment of grating marks on a photosensitive substrate.
It is still another object of the present invention to solve the conventional problems in photoelectric detection described above and to provide a position detection method or apparatus almost free from the influence of a surface state of a substrate such as a wafer.
It is still another object of the present invention to provide a position detection method or apparatus almost free from the influence of a light intensity difference between wavelength components even if a grating pattern (mark) is illuminated with an illumination light beam including a plurality of wavelength components.
It is still another object of the present invention to provide a high-precision positioning (alignment) apparatus which reduces a grating pattern position measurement error depending on the state of a substrate surface when the position of a grating pattern is to be measured upon irradiation of the illumination light beam having a plurality of wavelength components on the substrate.
Further, the present invention has as an object thereof to provide a position detecting method or apparatus which is hardly affected by the asymmetry (a variation in the amplitude reflectance) of the cross-sectional structure of a grating pattern (mark) even when the grating pattern is illuminated with an illuminating light beam of a single wavelength.
Furthermore, the present invention has as an object thereof to provide a highly accurate alignment apparatus which is reduced in the position measurement error of a grating pattern depending on the state of the surface of a substrate when the position of the grating pattern is measured by the use of an illuminating light beam of a single wavelength.
In order to achieve the above objects of the present invention, the present invention is applied to a method or apparatus in which a pair of coherent light beams are incident on a position detection grating mark (MG) at incident angles symmetrical, with respect to the pitch direction of grating marks, the grating mark (MG) being formed with a small step difference structure on the surface of a plate-like object (W) such as a semiconductor wafer or glass plate, and the position of the grating mark in the periodic direction of grating marks is detected on the basis of signals obtained by photoelectrically detecting changes in light amounts of diffracted light components from the grating mark in specific directions.
According to the present invention, coherent light beams (xc2x1LF) for symmetrically irradiating a grating mark are formed into n pairs of light beams (xc2x1D1n) having n (n=3 or more) different wavelengths xcex1,xcex2, xcex3, . . . , xcexn, and when the magnitudes of the wavelengths satisfy xcex1 less than xcex2 less than xcex3, . . . ,  less than xcexn, the n wavelengths or a plurality of coherent light sources (LS1, LS2, LS3) are selected to approximately satisfy the following relation within the range of about xc2x110%:
(1/xcex1xe2x88x921/xcex2)=(1/xcex2xe2x88x921/xcex3)= . . . =(1/xcexnxe2x88x921xe2x88x921/xcexn)
In a preferable arrangement of the present invention, the two light beams constituting a pair of multi-wavelength coherent light beams are heterodyned to have a predetermined frequency difference between them. A light beam obtained by intensity-modulating, with a beat frequency, a mutual interference light beam of two first-order diffracted light components generated from the grating mark (MG) in a direction perpendicular to the grating pitch direction is photoelectrically detected.
In a preferred application of the present invention, a multi-wavelength position detection apparatus is incorporated in a projection exposure apparatus as a TTR, TTL, or off-axial alignment means (mark detection means).
In order to solve the problem on photoelectric detection, the present invention is applied to a method of projecting an illumination light beam on a diffraction grating (MG) formed on a substrate (a wafer W or a fiducial mark plate FG) subjected to position detection, and photoelectrically detecting diffracted light components from the diffraction grating (MG), thereby detecting the position of the substrate. First, (a) illumination beams (e.g., beams xc2x1D11 and xc2x1D22 diffracted by a reference grating RG) having different wavelength components (xcex1 and xcex2) are projected on the diffraction grating (MG) to generate a plurality of diffracted beams having the respective wavelength components from the diffraction grating (MG); and (b) a first interference beam formed by interference between two diffracted beams having an order difference (+1st and xe2x88x921st orders; or 0th and 2nd orders) and having a first wavelength component (xcex1) of the plurality of generated diffracted beams is received by a first photoelectric element, and a second interference beam formed by interference between two diffracted beams having an order difference (+1st and xe2x88x921st orders; or 0th and 2nd orders) and having a second wavelength component (xcex2) of the plurality of generated diffracted beams is received by a second photoelectric element. Subsequently, (c) first position information (xcex94xcexX1) associated with the periodic direction of the diffraction grating is calculated by a circuit unit (CU3) on the basis of a photoelectric signal (Im1) from the first photoelectric element, and second position information (xcex94X2) associated with the periodic direction of the diffraction grating (MG) is calculated by a circuit unit (CU4) on the basis of a photoelectric signal (Im2) from the second photoelectric element. Finally, (d) the weighted mean of the first position information and the second position information is calculated by a circuit unit (CU5) by changing weighting factors in accordance with the amplitude value of the photoelectric signal from the first photoelectric element and the amplitude value of the photoelectric signal from the second photoelectric element, thereby confirming the position of the substrate on which the diffraction grating is formed.
Positioning, i.e., position measurement marks formed on the surface of a wafer or the like are generally formed on the surface with a small step difference. However, these marks have slight asymmetry depending on the wafer process such as etching and sputtering in the semiconductor processing or a coating irregularity in the photoresist layer. This asymmetry causes degradation of accuracy of mark position detection.
In the interference alignment method of photoelectrically detecting the mutual interference light beam of the two diffracted light components generated by the grating mark and utilizing the resultant photoelectric signal, the asymmetry of the grating marks causes asymmetry of the amplitude reflectances of the marks themselves, resulting in degradation of position detection precision. More specifically, if a difference occurs in the depths of the bottoms of the grooves of the lines constituting the grating marks or the thickness of the resist layer locally varies, the absolute values and phases of the amplitude reflectances of the marks themselves become asymmetrical in accordance with the changes in depth of the bottom of the groove and thickness of the resist. As a result, the intensity and phase of the positive-order diffracted light component generated from the grating mark in the right direction relative to the zero order light are different from those of the negative-order diffracted light component generated in the left direction relative to the zero order light. In this case, the intensity difference does not almost cause degradation of position detection precision, but the change in phase greatly affects the position detection precision.
The simulation results of position detection precision in the heterodyne scheme using an illumination light beam having a single wavelength as in the conventional case will be described with reference to FIGS. 1 and 2. This simulation is performed under an assumption that two coherent incident beams having a predetermined frequency difference are irradiated from two symmetrical directions on a grating mark MG on a wafer covered with a resist layer PR, as shown in FIG. 2, and the results are obtained by observing the state (e.g., an amplitude and a phase) of a mutual interference beam, i.e., an interference beat light beam, of the xc2x11st-order diffracted light components generated from the grating mark MG in a direction perpendicular to the surface of the grating mark MG while the wavelength is changed.
FIG. 2 illustrates a one-dimensional grating MG on a wafer or the like which is assumed in the simulation and the enlarged section of a grating portion with the resist layer PR coated on the surface of the grating. In this case, a pitch Pmg of the grating MG was set to be 8 xcexcm, a duty was set to be 1:1, the step difference (depth) T2 of a groove was set to be 0.7 xcexcm, and an asymmetry of 0.1% having a taper (inclination) xcex94S in the pitch direction was set in the bottom portion of the grating MG. A thickness T1 of the resist layer PR which covered the grating MG from the surface of the top portion of the grating MG was set to be 0.9 xcexcm, and a recess amount xcex94T on the resist layer surface corresponding to the position of each bottom portion of the grating MG was set to be xcex94T≅0.3 T2 (0.21 xcexcm). The grating structure shown in FIG. 2 is called a grating having an asymmetrical amplitude reflectance.
FIG. 1 is a graph obtained when a wavelength xcex (xcexcm) of an illumination light beam or an interference light beam obtained by synthesizing xc2x11st-order diffracted light components is plotted along the abscissa, and the relative amplitude of a change (AC component) of a signal corresponding to a change in light amount of the interference light beam and a position detection error amount (xcexcm) are plotted along the ordinate. In the simulation results shown in FIG. 1, the conditions for the grating mark structure and the resist layer in FIG. 2 were set such that the wavelength xcex for outputting the zero AC component, i.e., only the DC component of the photoelectric signal corresponding to the interference light beam received by the heterodyne scheme was 0.663 xcexcm as the wavelength of the Hexe2x80x94Ne laser.
As can be apparent from the above description, when a laser beam having a wavelength of 0.663 xcexcm is used, it is found that the mark position detection error near (about xc2x120 nm) this wavelength becomes very large. This is natural in the heterodyne scheme. If the AC component corresponding to the beat frequency is not included in a photoelectric signal subjected to phase difference measurement, the phase difference measurement is impossible. This is also true for homodyne position detection under the same conditions as described above for the grating mark structure and the resist layer.
The amplitude reflectance of the grating mark itself also greatly varies depending not only on the depth of the mark and the thickness of the resist but also the wavelength components of an illumination light beam. For this reason, the variation in amplitude reflectance of the grating mark is dependent on time coherence of the illumination light beam.
According to the first aspect of the invention for solving the problem on the illumination light beam side, to reduce the temporal coherence of the illumination light beam within a practical range with respect to the actual step difference structure of the marks and the actual state of the thickness of the resist layer and to obtain an incoherent illumination light beam, an assumption is given such that a plurality of coherent light beams having three or more different center wavelengths xcex1, xcex2, and xcex3, . . . , xcexn are used to obtain multi-wavelength light beams.
According to the first aspect, as the basic condition for incoherence, when sets of coherent light beams adjacent to each other in the order of wavelengths are taken into consideration, the respective wavelengths are determined such that a difference between wave number values (1/wavelength) of two light beams obtained for each set falls within the error range of about xc2x110%.
The temporal coherence of a multi-wavelength illumination light beam obtained using three coherent light beams (the wavelength widths are very narrow) having center wavelengths xcex1=0.633 xcexcm, xcex2=0.690 xcexcm, and xcex3=0.760 xcexcm, respectively is simulated. The results are shown in FIG. 3. The thickness (xcexcm) of a resist layer formed on a silicon substrate is plotted along the abscissa of FIG. 3, while the reflectance obtained as an intensity sum of reflected light components having a wavelength generated by the silicon substrate upon irradiation of a multi-wavelength illumination light beam is plotted along the ordinate of FIG. 3. The amplitudes of reflectance variations with changes in resist thickness represent the temporal coherence of the illumination light beam. This temporal coherence can also be obtained by Fourier transform of the spectral distribution of the illumination light beam.
According to the simulation results in FIG. 3, the variation amplitudes of the reflectances are found to be small and the temporal coherence is also found to be small when the resist thicknesses (or mark step differences) fall within the range of 0.5 xcexcm to 1.7 xcexcm. That is, an almost incoherent state is obtained. In this manner, the coherence can be reduced within the range of specific optical thicknesses and small step differences (i.e., the resist thicknesses fall within the range of 0.5 xcexcm to 1.7 xcexcm) because the basic conditions representing the relations of the wavelengths used to form a multi-wavelength light beam are satisfied within a deviation of xc2x110%.
More specifically, if the wavelengths xcex1, xcex2, and xcex3 are given as 0.663 xcexcm of the Hexe2x80x94Ne laser beam, 0.690 xcexcm of the beam of a commercially available semiconductor laser, and 0.760 xcexcm of the beam of another commercially available semiconductor laser, respectively, the following relations are established:
xcex94xcex12=1/xcex1xe2x88x921/xcex2=0.1305
xcex94xcex23=1/xcex2xe2x88x921/xcex3=0.1335
and their deviation is given as xcex94xcex23/xcex94xcex12=1.023 (an error of 2.3%). When a multi-wavelength illumination light beam is obtained from three light beams having wavelengths falling outside the above ranges, it is difficult to obtain a good incoherent state with a small variation amplitude of the reflectance within a specific range.
Condition xcex94xcex12≅xcex94xcex23 . . . ≅xcex94xcex(nxe2x88x921)n (xc2x110%) defined in the first aspect is satisfied to obtain an almost incoherent state within a practical range. For this reason, even if the resist thickness or the depth of the mark step difference slightly changes within the incoherent range (e.g., 0.5 xcexcm to 1.7 xcexcm), the mark detection position obtained by photoelectrically detecting a multi-wavelength interference light beam obtained from the xc2x11st-order diffracted light components generated from the mark rarely varies by the averaging effect of the multi-wavelength light beam obtained using beam having three or more wavelengths.
When a multi-wavelength light beam is obtained from coherent light beams having three or more wavelengths satisfying the conditions defined by the first aspect, the influence of asymmetry caused by variations in mark step difference and resist thickness is almost eliminated, and good position detection precision can be always maintained.
As previously described, the amplitude reflectance of the grating mark itself greatly varies depending not only on the mark depth and resist thickness but also the wavelengths of the illumination light beam (detection light beam). The detection light beam has a plurality of wavelengths (broad band), the amplitude reflectances of the mark itself are different in units of wavelength components, and position detection results are different accordingly. When the amplitude reflectances of the mark itself are assumed under various mark conditions, the position detection precision can be simulated.
The second aspect of the invention is based on the simulation results obtained as follows. Assume that a grating mark is to be irradiated with an illumination light beam having only a specific wavelength, and that diffracted light components generated by this grating mark are to be photoelectrically detected. If the intensity (i.e., the amplitude of a level change in signal during relative scanning between the illumination light beam and the grating mark) of a photoelectric signal becomes extremely low, the position detection precision is also degraded. According to the simulation results, in the second aspect, even under the grating mark condition that use of an illumination light beam having a single wavelength causes an extreme reduction in amplitude of a photoelectric signal, an averaged mean of the position detection results for each of the wavelengths using an illumination light beam having another wavelength is obtained to prevent extreme degradation of the position detection precision.
Even under the conditions shown in FIG. 1 or 2, when a semiconductor laser generates an illumination beam having a wavelength xcex of 0.670 xcexcm or 0.725 xcexcm, the mark position detection error can be sufficiently reduced. Judging from this fact, it is effect to use two-color illumination beams having different wavelengths, such as an Hexe2x80x94Ne laser and a semiconductor laser and give an attention (selection or weighting) to a mark position (position offset amount) detected upon beam irradiation having a wavelength which conducts a large amplitude in the detected signal (AC component).
Alternatively, another method is also available in which only an interference light beam of two 1st-order diffracted light components propagating in one specific direction is not detected, but an interference light beam of the 0th- and 2nd-order diffracted light components propagating in another direction is photoelectrically detected, and the mark position determined on the basis of the photoelectric signal is taken into consideration. FIG. 4 shows generation of 0th-, xc2x11st-, and xc2x12nd-order diffracted light components under the beam incident conditions that two irradiation beams xc2x1L1 having a wavelength xcex1 and two irradiation beams xc2x1L2 having a wavelength xcex2 are incident on a diffraction grating mark MG, and interference fringes having a pitch Pif and the same intensity distribution on the grating mark MG at the wavelengths xcex1 and xcex2 when the pitch Pmg of the grating mark satisfies relation Pmg=2Pif.
In FIG. 4, an interference beam BM of the 1st-order diffracted components xc2x1D1n propagating in a direction perpendicular to the surface of the grating mark MG has wavelengths xcex1 and xcex2. The 0th-order diffracted component (normal reflected light component) derived from the beams xc2x1L1 has an incident angle slightly different from that of the beams xc2x1L2. For this reason, four beams xc2x1D01 and xc2x1D02 corresponding to the beams xc2x1L1 and xc2x1L2 propagate in different directions. The first suffix in D01 and D02 indicates the diffraction order, and the second suffix indicates the wavelength (xcex1 or xcex2).
The 2nd-order diffracted light component xe2x88x92D21 generated upon irradiation of the beam +L1 propagates in a direction opposite to that of the optical path of the beam +L1 and interferes with the 0th-order diffracted light component +D01 of the beam xe2x88x92L1. Similarly, the remaining 2nd-order diffracted light components +D21, xe2x88x92D22, +D22 propagate in the same direction as that of the corresponding 0th-order diffracted light components xe2x88x92D01, +D02, and xe2x88x92D02. The intensities of the interference light beams between these 0th- and 2nd-order diffracted light components change in accordance with relative changes between the grating MG and the interference fringes as in the interference beam BM of the xc2x11st-order diffracted light components.
Assume only the wavelength xcex1. The 1st-order component (i.e., the interference beam BM of the 1st-order diffracted light components xc2x1D11) is photoelectrically detected to obtain the mark position (or position offset). At the same time, two 2nd-order components (i.e., the interference light beam of the 0th-order diffracted light component +D01 and the 2nd-order diffracted light component xe2x88x92D21, and the interference light beam of the 0th-order diffracted light component xe2x88x92D01 and the 2nd-order diffracted light component +D21) are photoelectrically detected. A value obtained by averaging the mark positions respectively obtained using the signals having these two 2nd-order components is defined as the position of the mark. The mark position detected using the 1st-order component or the mark position detected using the 2nd-order components is selected in accordance with the magnitude between the average of the amplitude values of the 1st-order component signals and the average of the amplitudes of the 2nd-order component signals. Alternatively, a weighted mean is obtained.
As described above, the order of the diffracted light components used for mark detection is changed because the directions of the diffracted light components generated by the grating MG are different depending on the order. Even if the amplitude of the change in intensity of an interference light beam of a given order component propagating in a given direction becomes small to degrade the detection precision, the amplitude of the change in intensity of an interference light beam of another order propagating in another direction does not become small, thereby preventing degradation of the detection precision.
This can also be confirmed from the simulation results shown in FIGS. 5A and 5B. FIGS. 5A and 5B are simulation graphs showing the relationships between the amplitudes of changes in signals (AC components) and the position detection errors using the step difference T2 of the grating MG in FIG. 2 as a parameter when the Hexe2x80x94Ne laser generates an irradiation beam having a wavelength of 0.633 xcexcm. In these graphs, pitch Pmg=8 xcexcm, duty=1:1, taper amount (ratio) xcex94S=0.1%, and the thickness T1 of the resist layer PR on the top surface is set to be 1.15 xcexcm. FIG. 5A shows the simulation for the interference beam BM of the 1st-order diffracted light components (1st-order diffracted light components xc2x1D11), while FIG. 5B shows the simulation for the 2nd-order (0th-order diffracted light components xc2x1D01 and 2nd-order diffracted light components xc2x1D21) interference beams.
As can be understood from FIGS. 5A and 5B, the amplitude components of the signals obtained by photoelectrically detecting the interference light beams of the 1st- and 2nd-order diffracted light components greatly change with a small change in the shape (step difference T2) of the grating mark. For example, in FIG. 5A, when the grating step difference T2 is 0.86 xcexcm, the amplitude of the change in intensity of the 1st-order diffracted components becomes very small, and the position detection error is abruptly increased accordingly. However, in FIG. 5B, when the step difference T2 is 0.86 xcexcm, the change in intensity of the 2nd-order interference light beam is relatively large, and degradation of the position detection error is small. Note that the amplitudes of changes in signals in FIGS. 5A and 5B are expressed as relative values, but the scales in FIGS. 5A and 5B are the same.
When an algorithm using both grating mark position detection using the interference light beam of the 1st-order diffracted light components and grating mark position detection using the interference light beam of the 2nd-order diffracted light components and employing one of the results, a weighted mean of the detection positions (or positional offsets) obtained from the illumination light beam having a plurality of wavelength components may be preferably obtained using the wavelength dependence, as can be apparent from the simulation in FIG. 1.
The detection light has a plurality of wavelengths, and the pieces of mark position information obtained in units of wavelength components are averaged to perform higher-precision position detection than the conventional case. As shown in FIG. 1, according to the simulation results, when the amplitude of a change (AC component) of a light amount signal of a diffracted light beam (interference light beam) having a given wavelength is small, a probability of degrading the position detection precision using the diffracted light beam having this wavelength is high. In detecting diffracted light beams (interference light beams) of a plurality of wavelength components, the mark positions detected in units of wavelength components are weighted with small weighting factors for small amplitudes of changes in signals, and large weighting factors for large amplitudes of changes in signals. The weighted components are then averaged. With this operation, the detection result of the mark position using a diffracted light component having a wavelength component having a high probability of a large error is automatically weighted with a small weighting factor. Therefore, the precision of the final mark position detection result can be maintained to a certain degree.
In detecting a 2nd-order (an interference light beam of the 0th- and 2nd-order diffracted light beams) signal, in order to individually obtain the mark positions using the signals photoelectrically detected in units of wavelength components, the failure in detection of the mark positions caused by the canceling effect of the respective wavelengths (to be described later) at the time of reception of the diffracted light beams (interference light beams) can be prevented.
According to the present invention, the incident beams of the respective wavelengths are irradiated while being sequentially switched one by one in grating mark detection. For this reason, a photoelectric element for photoelectrically detecting an interference light beam of the 1st-order diffracted light components and a photoelectric element for photoelectrically detecting an interference light beam of the 2nd-order diffracted light components need not be prepared for a plurality of sets. In addition, a wavelength selection means for discriminating the interference light beams in units of wavelength components can be omitted.
In order to solve the problem when the position of the grating pattern is measured by the use of an illuminating light beam, the present invention premises that the position or the position offset amount of a substrate on which a one-dimensional or two-dimensional diffraction grating-like pattern is formed is detected by the use of a coherent illuminating beam (light of a single wavelength). Further, the present invention is constructed so as to be also applicable to a position detecting apparatus of any of the conventional homodyne type and the conventional heterodyne type by some improvement. So, as an example, the epitome of the invention of an apparatus (method) for projecting two illuminating beams onto a grating pattern at symmetrical angles of incidence to thereby detect the position of the grating mark will hereinafter be described with reference to FIGS. 49 and 52.
First, in the present invention, provision is made of beam applying means (LS, TBO, AP, G1, G2) for applying two coherent illuminating beams (+LF, xe2x88x92LF) to a diffraction grating-like grating pattern (MG) formed on a substrate (W) of which the position is to be detected, at symmetrical angles of incidence. Provision is further made of a first photoelectric detector (DT1) for receiving a first interference beam (BM) obtained by the interference between two diffracted lights (xc2x11st-order diffracted lights) travelling in a first direction (vertical direction) from the grating pattern (MG), and second photoelectric detectors (DT2a, DT2b) for receiving second interference beams (BmO2, Bm2O) obtained by the difference between two diffracted lights (0-order and 1st-order diffracted lights) travelling in a second direction differing from the first direction from the grating pattern (MG).
Further, by scanning means such as a stage (WST) for moving the substrate (W) or an optical system TBO for creating two beams which provides a predetermined frequency difference between the two illuminating beams, the grating pattern (MG) is controlled so as to be relatively scanned by an interference fringe produced by the two illuminating beams (+LF, xe2x88x92LF).
As shown in FIG. 52, provision is also made of weight coefficient calculating means (amplitude ratio detection circuit 58) for calculating a first weight coefficient (C11) conforming to the ratio between the amplitude value of a first photoelectric signal (Im1) outputted from the first photoelectric detector (DT1) during the relative scanning and the substantially ideal amplitude value of the first photoelectric signal, and calculating a second weight coefficient (C22) conforming to the ratio between the amplitude value of second photoelectric signals (ImO2, Im2O) outputted from the second photoelectric detectors (DT2a, DT2b) and the substantially ideal amplitude of the second photoelectric signals.
Provision is further made of position calculating means (detection circuit 56) for calculating first position information (xcex94X11) for specifying the position of the grating pattern (MG) on the basis of the relation between a level change in the first photoelectric signal and a position which provides the reference of the relative scanning (or the reference in terms of time) and calculating second position information (xcex94X22) for specifying the position of the grating pattern (MG) on the basis of the relation between a change in the level of the second photoelectric signals and a position which provides the reference of the relative scanning (or the reference in terms of time), and weighted mean calculating means (60) for weight-averaging the first position information and the second position by the first weight coefficient and the second weight coefficient and determining the most apparently certain position (xcex94X) of the grating pattern (MG).